1. Field of the Invention
The present invention relates to a black-level feedback device that controls an output of a black-level period of a photoelectric conversion element to an optional device, an image reading device such as an image scanner, a digital copying machine, and a facsimile machine, and also relates to a black-level feedback control method.
2. Description of the Related Art
Among various kinds of image reading devices such as an image scanner, a digital copying machine, and a facsimile machine, there are some devices that sample image data of a light non-incident part of charge-coupled devices (CCDs) as photoelectric conversion elements, and set a concentration level as a black reference level, at the time of carrying out a quantization (for example, see Japanese Patent Application Laid-open No. H6-14188).
Further, among conventional image reading devices, there are devices that carry out image signal processing including a black-level correction process as shown in FIG. 21. As shown in FIG. 21, a CCD 201 reads an image of a document, and outputs an analog image signal to a signal processing integrated circuit (IC) 202. The signal processing IC 202 includes a sample holding circuit 203, a programmable gain amplifier 204, an analog-to-digital (AD) converter 205, and black-level correcting circuits 206 to 209. The sample holding circuit 203 samples an analog image signal from the CCD 201, based on a sample pulse, and holds a sample level, thereby forming a continuous analog image signal. The programmable gain amplifier 204 amplifies the continuous analog image signal at a predetermined gain. The AD converter 205 converts this signal into digital data. The digital data obtained by conversion by the AD converter 205 is supplied to the latter stage for processing, and is also feedback to the black-level correcting circuits 206 to 209.
The feedback processing carried out by the black-level correcting circuit is explained. A black average processor 206 averages assigned pixels of image data in one line during a black level period or a period corresponding to a black level (hereinafter, a result of the average processing is described as a black detection level). The black average processor 206 calculates a difference (hereinafter, “black differential”) between a black-level target value 207 determined in advance and the black detection level. The black average processor 206 converts a black differential value into a digital-to-analog (DA) converter code by a proportion set in advance by a digital-to-analog-conversion (DAC) set value calculator 208, and reflects the DA converter code to a black correction DAC 209. This processing is carried out for each one line, thereby converging the black detection level to the black-level target value 207. The above series of a black correcting circuit operation are hereinafter described as a black-level feedback control.
FIG. 22 is a schematic of an image format of image data output from a CCD. XLSYNC is a cycle line of one line. After XLSYNC=“L”, the CCD 201 outputs image data in the order of a preliminary-feeding image, an optical black (OPB) image, and a valid image. Both the preliminary-feeding image and the OPB image are black-level image data of the CCD 201. BLKCLP becomes active during the black level period of the CCD 201. The black average processor 206 averages the assigned pixels during an “H” period, and establishes a black detection level.
According to the above conventional example, when the pixel density becomes higher, and also when the image reading speed becomes faster, this increases a clock frequency for driving the photoelectric conversion element such as the CCD 201 which reads images and the signal processing IC 202 which carries out various processing to the image signals output from the photoelectric conversion element at the latter stage. Accordingly, unnecessary radiation of electromagnetic waves increases.
To prevent this inconvenience, a spread spectrum clock generator (SSCG) as a frequency spreader is provided inside or at a latter stage of an oscillator which is used to generate a clock frequency, thereby decreasing the strength of the unnecessary radiation of a peak part of the frequency. In other words, as shown in FIG. 23, the SSCG spreads the clock frequency of a spectrum characteristic S1 to obtain a spectrum characteristic S2, thereby decreasing the strength of the unnecessary radiation to a level smaller than that of the spectrum characteristic S1 of the frequency before the spreading.
However, when the SSCG is used for a driving clock of an analog system, an image signal level cyclically changes in one line even when the same concentration level is read, due to a variation of an output waveform of the CCD 201 and a variation of a sample timing of the signal processing IC 20. Therefore, high and low levels of the image occur.
This problem is explained with reference to FIG. 24. In general, a CCD driving signal is generated from a high-precision reference clock having oscillation precision of 50 parts per million and 100 parts per million, for example. When the frequency is spread to avoid unnecessary radiation, the frequency changes along lapse of time, as shown in the lower part of FIG. 24, where the lateral axis represents time and the vertical axis represent frequency. In other words, the frequency is structured to change smoothly, in a predetermined width of ±0.5% and ±1.0% around a reference frequency. Usually, the frequency has a regular frequency spread cycle (frequency cycle). As shown in the lower part of FIG. 24, the frequency changes by a predetermined modulation width to a direction (a high-frequency side: + side) in which the clock cycle becomes short relative to the reference frequency. After this, the frequency changes by a predetermined modulation width to a direction (a low-frequency side: − side) in which the clock cycle becomes long along the same characteristic curve. The frequency repeats this modulation cycle, and returns to the reference frequency. Accordingly, the reference frequency and the phase match together at every one half cycle of the modulation cycle. The upper part of FIG. 24 represents an image level variation relative to the modulation cycle. It is understood from this graph that the image level changes in synchronism with the modulation cycle, where the lateral axis represents time and the vertical axis represent an image level. There is also a system in which the modulation cycle changes at random or following a specific rule.
When the above variation of the image level is repeated in many lines, high and low levels occur as fine strings in the read image, and these fine strings appear as lateral strings in human eyes. In other words, a variation of a sub-scanning image level occurs due to the main-scanning image level variation.
A process in which the sub-scanning image level variation occurs is explained with reference to FIG. 25. FIG. 25 depicts a main-scanning black level variation in six lines. The modulation cycle of the SSCG is not synchronized with one line cycle of the device. Therefore, the black level variation has a different phase for each line. In FIG. 25, the phases of the black-level variation match in five-line unit. To average the black levels during the BLKCLP=“H” period to obtain a black-level target value, the black-level feedback control is carried out, as described above. However, in the example shown in FIG. 25, the black-level average values detected for each line are different. Because the black-level feedback control is carried out to the different black detection levels to obtain a black-level target value, the offset of the black level varies for each line. Therefore, the sub-scanning offset variation of the same pattern occurs in five lines. This sub-scanning offset variation appears as a thick lateral string in the variation line cycle unit. In the human eyes, the image variation in a constant width tends to be most noticeable. Therefore, rather than the lateral-string image due to the main scanning variation, the lateral-string image due to the sub-scanning variation becomes a problem.
In other words, because the image level variation according to the SSCG has no change in the center level (the black-level center shown in FIG. 25), this image level variation should not basically be corrected by the black-level feedback operation. A variation that needs to be corrected by the black-level feedback operation is the image level variation of which center level changes. For example, there is a phenomenon called a smear in which a black level in a white region is raised when the white region and the black region of a document are being read. In this case, because the center level of the image level variation changes, the image level variation needs to be corrected by the black-level feedback operation.